Friday, 21 July 2017

NGC6537: the Red Spider

Planetary nebulae are fascinating objects and exist in a seemingly infinite number of varieties, as many as there are dying stars. The Red Spider Nebula (NGC6537), for instance, is definitely one of the more eccentric planetaries but unfortunately requires quite a bit of telescope and very good skies to be admired fully. Photographs clearly show its bipolar structure and four "legs" that extend up to a hundred billion kilometres away from the central star. A bipolar structure is certainly not uncommon in planetary nebulae because often the matter outflow is obstructed at the star's equator by its greater density there, forcing the gas to blow out via the poles. The peculiar shape of the Red Spider, however, leads scientists to believe that there must be a small companion star nearby which distorts the nebula formation.

It's clearly still a very young nebula in full expansion, with complex and turbulent gas structures that are being hurled into space by stellar winds up to 300 km/s. Another odd thing is that it appears red in stead of greenish-blue, an indication of a high presence of ionised nitrogen, although there's also a second explanation. The Red Spider lies in one of the densest parts of our Milky Way, towards its nucleus, and is surrounded by thick clouds of interstellar dust. These clouds work like sunglasses, i.e. they not only dim the light from the nebula considerably, but also change its colour towards the lower end of the frequency range (red). Without those clouds the Red Spider would shine 40 times brighter and would easily be within reach of small telescopes. Unfortunately, even with my big binoscope I was only able to distinguish a hint of its four famous "legs"...